A lubricating coating is usually formed on the surface of a metal, such as iron, steel, or stainless steel, that will be subjected to plastic working in order to prevent the scuffing and galling that would be produced by metal-to-metal contact between the workpiece and tool. One type of lubricating coating that can be produced on the metal surface involves the physical attachment of lubricant to the metal surface. Another type involves the application of lubricant after the production of a conversion coating on the metal surface by a chemical reaction in a preliminary conversion treatment. Physical attachment of the lubricant on the metal surface provides a poorer adherence than use of the lubricant on a conversion coating elaborated on the metal surface, and as a result the former type is usually employed with light working processes. A phosphate coating or oxalate coating is produced on the metal surface when a conversion coating is used; this conversion coating functions as a carrier. A sliding lubricant is applied after the production of this conversion coating. This type, which has a two-layer structure of conversion coating (=carrier film) and lubricant, exhibits a very high resistance to galling and as a consequence has been used over a very wide range of applications in the plastics working sector for wire drawing, tube drawing, and forging. Within the realm of plastic working operations, the application of lubricant on an underlayer of a phosphate or oxalate coating is frequently used in particular in the severe working sector.
The lubricants applied on conversion coatings can also be broadly divided into two types depending on the method of application. One type involves physical attachment of the lubricant on the conversion coating, while the other type involves a reactive attachment of the lubricant on the conversion coating. The first type includes lubricants comprising an extreme-pressure agent added to a mineral, vegetable, or synthetic base oil. The first type also includes lubricants comprising a solid lubricant (e.g., graphite or molybdenum disulfide) dissolved along with a binder component in water; these lubricants are attached by drying. Since these lubricants can be simply applied by spraying or immersion, they offer the advantage of making bath management almost unnecessary. However, due to their low lubricity they are frequently used in relatively light lubricating environments. With regard to the second type, treatment is carried out with a reactive soap, such as sodium stearate, functioning as the lubricant. Reactive soaps are used as the lubricant in particular when a high lubrication performance is required. The reactive soaps have a high lubrication performance due to their reaction with the conversion coating.
The use of a reactive soap, however, requires bath management since a chemical reaction is involved, temperature management in order to control the chemical reaction, and waste disposal and bath renewal due to bath deterioration. Reducing the amount of industrial waste has recently become a major issue in view of the goal of global environmental protection. This has created demand for a lubricant and treatment method that do not produce waste. The complexity of process and treatment bath management in the prior-art technology has also made the appearance of a simple process desirable.
In order to solve the problems identified above, Japanese Laid-Open (Unexamined or Kokai or A) Patent Application Number Sho 52-20967 (20,967/1977) discloses a “lubricant composition comprising solid lubricant, a conversion coating-forming agent, and a base of water-soluble polymer or water-based emulsion thereof”. This lubricant composition, however, does not match conversion coating treatments.
Another means for solving the problems under consideration is, for example, the invention in Japanese Laid-Open (Unexamined or Kokai or A) Patent Application Number Hei 10-8085 (8,085/1998), which is assigned to the present applicant. This invention relates to a waterborne lubricant for the cold plastic working of metals. This waterborne lubricant comprises (A) water-soluble inorganic salt; (B) solid lubricant; (C) at least one oil component selected from mineral oils, animal and vegetable fats and oils, and synthetic oils; (D) surfactant; and (E) water. The solid lubricant is present uniformly dispersed, while the oil is present uniformly emulsified. The lubricant provided by this invention is not stable in industrial applications due to the presence of the oil component in emulsion form, and this lubricant therefore does not provide a high lubricity on a stable basis.
Yet another means for solving the problems under consideration is, for example, the invention in Japanese Laid-Open (Unexamined or Kokai or A) Patent Application Number 2000-63880, which is assigned to the present applicant. This invention relates to a lubricant composition for the plastic working of metals. This lubricant composition contains (A) synthetic resin, (B) water-soluble inorganic salt, and water, wherein the solids weight ratio (B)/(A) is 0.25/1 to 9/1 and the synthetic resin is present dissolved or dispersed. The lubricant provided by this invention does not generate an acceptable lubricity on a stable basis under severe working conditions due to its use of synthetic resin as a main component.